Laser Jello
Use gelatin as a smoked lens, to view total internal reflection, and as a color filter.
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D-shaped refraction or petri dishes
One red-colored and one blueberry package or Jello™
A laser (preferably HeNe)
Make the gelatin according to the Jigglers™ recipe. Then, shine the laser through it
#1 Shine the laser through the red gelatin first. Notice that a beautifully visible beam travels
through it. Then shine the laser through the blueberry gelatin.
#2 Using the red gelatin and the curved side of the dish, shine the laser parallel to the central
axis. Move the laser from the center to the outside edge. Notice the change and measure the
angle of refraction.
#3 Turn the dish around. Shine the laser perpendicular to the straight edge. Move the laser to the
outside edge so that the beam reflects off the surface of the gelatin and is re-emitted parallel to
the incoming beam. (See title photo) Repeat the activity, bouncing the beam off the straight side,
to measure the critical angle.
#4 If you have two lasers, align them parallel and shine them through the curved side of the dish.
Use a white piece of paper or waxed paper as a screen to find the focal point. Invert the gelatin
lens onto a piece of waxed paper--it may need a little prying to get it to come out. Use the empty
dish as a cookie cutter to make a concave lens out of the original convex gelatin lens. Shine
parallel lasers through the new lens.
#1 Red objects appear red because they don't resonate at red frequencies. The red wavelength
energy is either reflected or transmitted. Gelatin is colloidal and scatters enough of the beam to
make it visible. The blueberry Jello™ is actually cyan, a mixture of blue and green (check it on
the overhead with a spectrum created by a diffraction grating). Cyan is the complementary color
of red. The blueberry color transmits green and blue, but changes red wavelengths into heat.
#2 As light enters the gelatin, the change in medium causes a change in speed. A ray of light will
refract similar to the way a car will change direction as it moves at an angle from asphalt to soft
dirt or loose gravel.
#3 As light tries to travel from a slower (or more optically dense) substance to a faster medium,
it may reflect similar to the skimming of a stone off the surface of water. If the beam hits at an
angle that is small relative to the surface , then the light will reflect; if the angle is closer to
perpendicular, then the beam will refract.
#4 Light traveling through the convex lens will converge. Light traveling through the concave
lens will diverge. This might be a good time to introduce the reversibility of light. Shine the light
through the jello lens. Mark its path into and then out of the jello. Then shine a laser backwards
along this path. That is, shine it into the path that the original beam exited. The light path
followed by the reversed beam will be exactly the same.